Développement de capteurs à fibre optique basés sur la résonance ...

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Article i Submitted to Thin Solid films Development of a Fiber-Optic Sensor Based on Surface Plasmon Resonance on Silver Film for Monitoring Aqueous Media Wen Bin LIN, Monique LACROIXa, Jean Marc CHOVELON2, Nicole JAFFREZICRENAULT*,, Alain TROIJILLETb, Colette VEILLAS", Henri GAGNAIREb a IFOS, Ecole Centrale de Lyon, 36 Avenue Guy de Collongue, BP163, 69131 Ecully Cedex, France b TSI, Université Jean Monnet, 23 Rue du Dr. Paul Michelon, 42023 Samt-Etienne Cedex, France Electronic Science Department, Nankai University, Tianjin, 30071, China * Corresponding author: Tel: +33 4 72186243; Fax: +33 4 78331577; Email:Nicole.Jaffrezic@ec-lyonfr Abstract: A reliable fiber-optic SPR sensor based on silvér is developed in this paper for chemical and biological applications. The range of measurable indices is dropped down by coating an overlay of zirconium acetate on the silver surface by sol-gel technique. The feasibility has been investigated in advance by theoretical analyses. The experimental fiberoptic sensor demonstrates its capability to operate in the aqueous media with the detectable range of refractive indices of 1.33-1.36. A self-assembled monolayer (SAM) of long chain thiol is introduced to cover the surface of silver in order to prevent silver from deterioration. Experimental studies demonstrate that, by this way, the lifetime of sensor increases from some days to some weeks. This work provides a base for developing an affinity biosensor. Key vords: Sensor; Surface plasmon; Optical coatings; Sol-gel technique; Self-assembled monolayer (SAM) 1. Introduction The fiber-optic based surface plasmon resonance (SPR) sensors have some advantages such as flexibility, low cost, small size and possible use in remote sensing over the traditional bulk optical systems [lj. Recently, a simple SPR multimode fiber-optic sensor using oblique
Article 1 Submitted to Thin Solid films injection of a parallel monochromatic light has been developed [2,3]. Silver is used to support SPR which is excited by light of 670nm wavelength emitted from a laser diode. However this fiber-optic sensor, with its measurable index range at about 1.35-1.40 [3], is still incapable of monitoring aqueous environment whose refractive index is 1.33-1.36. The limitation has prevented its applications in most practical chemical and biochemical systems. Two ways can be considered to drop down the range of measurable indices. One is to reduce the wavelength of light source, but the laser diodes commercially available with the wavelength of about 600-900nm do not give much choice. The other solution, as it is adopted in this paper, is to deposit an additional overlay on the surface of the metal film. Another problem confronted is the deterioration of silver. Gold and silver are known to be the most important metals for SPR applications [4]. Gold is physically and chemically stable while silver can provide the sharpest SPR. But the oxidation of silver happens as soon as exposed to air and especially to water, which gives rise much difficulty to bring about a reliable sensor for practical applications. A treatment of the silver surface by a thin and dense cover is therefore suggested. The theory in predicting the index-dependent thickness of an overlay is discussed firstly in this paper. A zirconium acetate (marked as ZrO2 in this paper) overlay is coated on silver surface as thick as some dozen nanometers by sol-gel technique. Lastly, the stability of the fiber-optic sensor is improved by introducing a self-assembled monolayer (SAM) of long chain acid thiol between the layers of silver and Zr02 in order to protect the silver film from oxidation. 2. Theory A relation between the thickness and refractive index of an overlay on four-layer sensor has been studied by W.J.H. Bender et al [5]. As it has been mentioned above, our transducer illustrated in figure 1 is a 5-layer structure that consists of fiber core, metal silver, thiol cover, Zr02 overlay and aqueous sample (see in figure 2). Since the thickness in total (less than one hundred nanometers) is rather small compared with its length (15mm) and the diameter of the core (600tm), the sensing section can be treated rather exactly as a one-dimensional multilayered planar configuration.
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